MEMBER-EQUAL

(Member-equal x lst) equals the longest tail of lst that
begins with x, or else nil if no such tail exists.

(Member-equal x lst) has a guard of (true-listp lst).
Member-equal has the same functionality as the Common Lisp
function member, except that it uses the equal function to
test whether x is the same as each successive element of lst.
See member and see member-eq.